The present invention relates to a process for producing a molded foam article of a crystalline aromatic polyester resin and, more particularly, to a process for producing a molded foam article used in industrial parts, food containers or the like, which is superior in various characteristics such as low density and heat resistance and has a good appearance.
An aromatic polyester resin represented by a polyethylene terephthalate has widely been used in the fields of electric/electronic parts, automotive parts, industrial parts, and packagings such as film and bottle containers because of its comparatively low price, excellent chemical properties such as chemical resistance, solvent resistance and weathering resistance, and physical properties such as heat resistance, rigidity and gas-barrier property. Therefore, trials to produce a molded foam article containing an aromatic polyester resin as a base resin, which is lightweight and has excellent insulating properties and cushioning properties, have been made. For example, Publication of Unexamined Patent Application No. JP 51-50365, A (1976) discloses a potentially foamable polyester fiber produced by impregnating an unstretched fiber, which is obtained by wet spinning or dry spinning of a high-melting point polyester, with a low-boiling point liquid which is insoluble or slightly soluble in the polyester. This publication also discloses that a polyester foam was obtained by heating the potentially foamable polyester fiber to the temperature higher than its plastication temperature. Publication of Unexamined Patent Application No. JP 59-135237, A (1984) discloses a foamed linear polyester resin, and also discloses that a food container capable of being heated in an oven can be obtained by forming a sheet of the foamed resin.
However, an object of the former publication is to finally obtain a thread-like foam by spinning a polyester to form a thread-like product, impregnating the product with a low-boiling point liquid as a blowing agent, and dipping in an oil bath, thereby to foam the product. Therefore, the former publication does not suggest a technical idea of processing the thread-like foam again to mold a molded foam article other than the thread-like product itself. The latter publication discloses that a foamed sheet is produced by treating a blend of a linear polyester/polycarbonate at high temperature and conducting extrusion foaming of the blend using carbon dioxide released from polycarbonate as a blowing agent to produce a foamed sheet. Only sheets having a density of 0.83 g/cm3 are described in the Examples of the publication. Therefore, a foam having a lower density is hardly obtained.
In Publication of Unexamined Patent Application No. JP 2-251543, A (1990), the present applicant has already suggested a process for producing a low density foamed sheet in an industrial manner, and has succeeded in practical application. As a result, it became possible to form the sheet into a lightweight container having a desired shape by a process such as vacuum forming and matched-mold forming using such a low-density foamed sheet.
As described above, various studies of thermally forming a foamed sheet produced by extrusion foaming to obtain a container such as food tray have been made. However, a so-called cavity-molding process of aromatic polyester resin prepuffs using a mold, followed by cooling and further removal from the mold to obtain a molded foam article having an arbitrary shape wherein the prepuffs are expanded and fused has never been studied sufficiently. The present applicant also has studied intensively about this process.
To obtain a molded foam article of a polystyrene resin by the cavity-molding process, the molded foam article is produced through the step of impregnating resin particles with a blowing agent, pre-expanding the resin particles so impregnated (so called prepuffs), and cavity-molding(expanding and fusing) the prepuffs. However, in case where these steps are applied to a polyester resin, a long time is required in the step of impregnating with the blowing agent or impregnation is not conducted because the polyester resin is superior in gas-barrier property. The crystallinity of the prepuffs is excessively high by heating on impregnation and pre-expanding and the prepuffs are not fused each other on cavity-molding and, therefore, a molded foam article is not obtained.
Thus, the present applicant has found that the crystallinity of prepuffs can be set to 25% or less by employing the step of melt-kneading a polyester resin and a blowing agent, conducting extrusion foaming of the mixture using an extruder, and cutting the resulting foamed extrudate to give prepuffs and that the prepuffs are expanded and fused each other by cavity-molding to obtain the desired molded foam article {Publication of Unexamined Patent Application No. JP, 8-174590, A (1996)}.
This molded foam article is free from gaps between prepuffs and has a useful heat resistance.
However, prepuffs made of a general purpose polyester resin having a crystallization peak temperature lower than 130xc2x0 C. is used in this molded foam article and, therefore, the crystallization rate is considered very fast. Therefore, the fusion rate is improved by controlling the crystallinity of the prepuffs to 25% or less, thereby making it possible to provide a molded foam article by fusing the prepuffs each other to some extent. However, the fusion ratio of prepuffs is 20% at most and a molded foam article having sufficient fusion ratio such as 30% or more could not be obtained.
Since the crystallinity of the molded foam article could be enhanced to 20% or more, the heat resistance can be imparted. However, since the fusion ratio is insufficient, the dimensional change on heating at 140xc2x0 C. for 24 hours is at least about 2.5% and could not be controlled to 2% or less.
Recently, a molded foam article whose dimensional change on heating at 140xc2x0 C. for 24 hours is controlled to 2% or less, preferably 1% or less, has been required for applications such as industrial parts and automotive parts, however the molded foam article could not respond to these requirements.
On the other hand, when the crystallization rate of a polyester resin is inhibited, for example, when the crystallization peak temperature is adjusted to 130-180xc2x0 C., the crystallinity of prepuffs can be reduced to 8% or less. Since the crystallinity is inhibited, proceeding of the crystallinity on cavity-molding can also be inhibited and the fusion of the prepuffs can be improved to excellent fusion ratio of not less than 40%.
When the crystallization rate is inhibited, the fusion can be improved. However, in case where the molded foam article is produced by a general process for cooling immediately after the completion of molding, removing the molded foam article from the mold, so-called sink wherein the center portion of the molded foam article shrinks immediately after removal from the mold, which is considered to be caused by low crystallinity of the molded foam article, occurs and the mature step of aging until sink is restored is required.
This sink is a phenomenon wherein the thickness of the plate-like molded foam article varies from the side portion to the center portion and the center portion is thin. This phenomenon is particularly remarkable in the prepuffs wherein the crystallization rate was inhibited. Therefore, even if the molded temperature is raised or the mold time is prolonged, to thereby to accelerate crystallization and to inhibit sink, melt marks are formed on the surface of the molded foam article, resulting in poor appearance. Therefore, it is difficult to produce a good molded foam article.
Accordingly, a mature period for several weeks until sink is restored after the production of the molded foam article of the aromatic polyester resin is required. Therefore, the mature period and place for maturing can cause an increase in cost.
If the above-described problems are solved, it is expected that the molded foam article of the aromatic polyester resin finds its way into a variety of applications such as building materials, construction materials, industrial members, automotive parts, etc., as a more highly functional material than that of conventional polystyrene or polyolefin products taking advantage of the excellent characteristics described above.
An object of the present invention is to provide a process for producing a molded foam article, which has sufficient fusion ratio such as about 30% or more and excellent dimensional stability capable of inhibiting the dimensional change ratio on heating, even in case of using prepuffs of the general purpose polyester resin. Such a molded foam article can be suitably used in applications such as industrial parts and automotive parts.
In this invention, in the case where cavity-molding is conducted by using prepuffs wherein the crystallization rate is inhibited, for example, the crystallization peak temperature is adjusted to within a range of from 130 to 180xc2x0 C., a molded foam article of an aromatic polyester resin can be produced wherein generation of sink, that is liable to occur in the molded foam article, is also inhibited.
Another object of the present invention is to provide a process for producing a molded foam article, wherein the above fusion, sink and appearance are improved and the heat resistance is imparted by accelerating the crystallinity within a shorter time, by adding a step of heating again to a specific temperature after the step of immediately following the molding.
The present invention relates to a process for producing a crystalline aromatic polyester resin molded foam article, which comprises molding crystalline aromatic polyester resin prepuffs using male and female mold members of a mold assembly through the following steps (1) to (4):
step (1) of filling a mold cavity, which is formed by closing the male and female mold members, with the crystalline aromatic polyester resin prepuffs;
step (2) of heating a surface of the mold to a temperature in a range of from (Tg+35) to (Tg+57)xc2x0C. (Tg is a glass transition temperature of the crystalline aromatic polyester resin prepuffs), thereby to mold the filled prepuffs;
step (3) of cooling the surface of the mold to a temperature not lower than Tg over a period of at least 20 seconds while holding the molded foam article in the mold as it is; and
step (4) of removing the molded foam article from the mold, after finally cooling the surface of the mold lower than Tg.
According to the process for the present invention, by providing step (3) of cooling the molded foam article to the temperature at which the surface temperature of the mold for cavity-molding is not lower than Tg for a period of 20 seconds or more without removing from the mold after the completion of heating for cavity-molding (expanding and fusing), a molded foam article having a sufficient fusion ratio of about 30% or more can be produced even when using prepuffs having very high crystallization rate equivalent to that of a general purpose polyester resin. The molded foam article whose fusion ratio was improved to 30% or more can satisfy the quality standard wherein the thermal dimensional change on heating at 140xc2x0 C. for 24 hours is lowered to 2% or less, which has been required in applications such as industrial members and automotive members.
The present invention also relates to a process employing the step of cooling once under the above-described specific conditions, there can be produced a molded foam article having a good appearance wherein generation of sink which is liable to occur in the molded foam article is also inhibited in case of molding prepuffs wherein the crystallization rate is inhibited, for example, the crystallization peak temperature is adjusted to within a range of from 130 to 180xc2x0 C.
In the process for producing a crystalline aromatic polyester resin molded foam article of the present invention, the addition of following step (3a) between steps (3) and (4) is effective:
step (3a) of heating the surface of the mold again to the temperature within a range of from (Tg+20) to (Tg+57)xc2x0C., accelerating crystallization of the molded foam.
The better process of the present invention, wherein the cooling of step (3) is conducted for a shorter time, for example from 20 to 300 seconds and the providing of the above step (3a) between the steps (3) and (4), the before described fusion is further improved and generation of sink is inhibited. At the same time, crystallization is accelerated within a shorter time, thereby making it possible to impart the heat resistance to the molded foam article having good appearance.